Digital information is commonly stored on data-storage disks. Data-storage disks are used in conjunction with some type of disk drive that is adapted to rotate the disk. Disk drives typically comprise a data-transducing head that writes and/or reads information to and from the disk surface as the disk rotates. More particularly, the head writes and/or reads information to and from a series of continuous tracks arranged concentrically about the geometric center of the disk.
The data-transducing head is typically disposed on some type of actuator mechanism. The actuator mechanism positions the head proximate the particular track to or from which data is being written or read, thereby allowing the head to perform its data-transducing function. Positions from which the head can write or read data are referred to throughout the specification and claims as "data transducing positions."
The actuator mechanism usually comprises an actuator arm, a flexure, a suspension arm, and a motor. The head is typically coupled to an end of the flexure via an air-bearing slider. An opposite end of the flexure is affixed to the suspension arm so as to cause the slider and the head to be disposed below the arm. The suspension arm is affixed to the actuator arm. The actuator arm is coupled to the body of the disk drive in a manner that facilitates rotational or, alternatively, linear movement in relation to the data-storage disk. The actuator arm is coupled to the actuator motor, which produces the force that moves the actuator arm. Movement of the actuator arm causes a cooperative movement of the data-transducing head due to the coupling of the head and the actuator arm by way of the suspension arm, the flexure, and the slider. The operation of the actuator motor and, thus, the position and velocity of the data-transducing head, are typically regulated by a servo controller.
The slider and the head are disposed proximate a surface of the data-storage disk when the head is in a data-transducing position, as noted above. The rotation of the disk produces an aerodynamic boundary layer on the surface of the disk. The slider is designed to interact with this boundary layer. More particularly, the shape of the slider causes the slider to be lifted by the boundary later. This lifting action causes the slider to fly above the disk surface at a height typically on the order of several micro-inches.
The actuator mechanism moves the head to a stored, or parked, position at the conclusion of read/write operations within the disk drive. The head may be parked on the surface of the data-storage disk, in an area that is not utilized for data storage. Alternatively, the head can be parked in a position away from the disk surface. The latter storage methodology prevents the slider and the head from contacting the surface of the data-storage disk at the start and finish of read/write operations. Specifically, parking the slider away from the disk prevents the slider and the head from dragging across the disk surface when the disk is rotating at a velocity insufficient to generate enough lift to support the slider. Drives that employ this storage methodology typically park the head in a location proximate the outer circumference of the disk. Parking the slider away from the disk is usually required in removable-media drives to facilitate insertion and removal of the data-storage disk.
Drives that park the data-transducing head away from the data-storage disk usually comprise a head-loading ramp. Head-loading ramps typically have an inclined surface and a flat. The suspension arm rests on the flat when the head is parked. The head-loading ramp is usually disposed proximate the outer circumference of the disk, with one end of the inclined surface overlapping the surface of the disk. The suspension arm is positioned on the ramp flat when the head is parked. The suspension arm slides down the inclined surface as the actuator moves the head from its parked position to a data transducing position. The sloped geometry of the inclined surface causes the gap between the head and the disk to undergo a gradual decrease as the head approaches the disk. Optimally, this decrease continues until the head assumes it normal flying height above the surface of the disk. The gradual decrease in the gap between the head and the disk provides an opportunity for the above-noted lifting force to develop between the air-bearing slider and the disk surface (this process is commonly referred to as "loading" the slider). Hence, under optimal circumstances, contact between the head and the disk does not occur during the loading process.
The ramp and the suspension arm function in a converse manner as the head is moved away from a data-transducing position. Specifically, the overlapping portion of the inclined surface slidably engages the suspension arm as the actuator moves the head toward the outer circumference of the disk. The sloped geometry of the inclined surface lifts the arm away from the surface of the disk. The motion of the suspension arm lifts the head by way of the flexure and the slider, thereby dissipating the lifting force between the slider and the disk surface (this process is commonly referred to as "unloading" the slider). The ramp continues to lift the arm and the head as the actuator moves the head away from the disk, until the head reaches its parked position.
The engagement of the head-loading ramp and the suspension arm generates a frictional force. This force is dependent upon the relative velocity between the ramp and arm. The frictional force is typically at its strongest prior to the point at which the suspension arm begins moving in relation to the ramp. The frictional force generated at this point is due primarily to static friction, i.e., friction between two non-moving surfaces. This type of friction is commonly referred to as "stiction." The frictional force between the ramp and the suspension arm undergoes an abrupt and substantial decrease as the arm begins moving. This decrease corresponds to a change in the type of friction acting between the ramp and the arm. More particularly, the frictional force generated by the slidable engagement of the ramp and the suspension arm is due primarily to coulomb friction, i.e., friction between two moving, non-lubricated surfaces. In general, the coulomb friction between two surfaces is substantially smaller than the static friction generated between the same two surfaces.
The motion of the data-transducing head as it moves between its parked and data-transducing positions is usually controlled by a servo controller, as noted above. These controllers typically employ a single, closed control loop to regulate the velocity of the head in relation to the data-storage disk. Precise control of the head's velocity is critical when the head is moved between the above-noted positions, as substantial velocity variations can cause the head to crash into the rotating data-storage disk. Such contact can damage the head and the disk, and can lead to a loss of data. Furthermore, substantial velocity variations as the head is being parked can cause the suspension arm to overshoot its parked position on the ramp, and can thereby damage the head, the slider, the flexure, or the suspension arm.
Variations in the velocity of the data-transducing head as it is moved between its parked and data-transducing positions can be caused by a number of factors. For example, variations may be caused by the effects of actuator inertia, aerodynamic interaction between the slider and the data-storage disk as the slider is loaded and unloaded, and changes in the slope of the head-loading ramp. In addition, changes in the above-noted frictional force between the ramp and the suspension arm exert a major influence on head velocity. In particular, the transition between static and coulomb friction as the arm begins to move generally results in a significant head-velocity change. Such changes can cause the head to undergo a velocity excursion that, due to the limitations of conventional closed-loop controllers, cannot be arrested in time to prevent the head from crashing into the data-storage disk.
Typical servo controllers regulate the velocity of the data-transducing head through the use of a corrective output based exclusively on a velocity error. More particularly, these controllers generate a velocity correction that is proportional to the difference between an actual (as measured) head velocity and a reference (desired) velocity. Some controllers also base the correction on the rate of change of this difference. Typical servo controllers are unable, however, to differentiate between the various sources that contribute to the velocity error. This inability represents a major drawback because, as explained in detail below, optimal head-velocity control requires that head-velocity corrections be tailored to the specific type of source that is responsible for the error.
The need for precise velocity control of the data-transducing head is becoming more critical due to current consumer-driven demands to reduce the form factor and data-access times of disk drives. More particularly, decreasing the form-factor of a drive necessitates reducing the footprints of the individual components within the drive. The footprint of a head-loading ramp can be reduced by increasing the ramp's steepness. Increased ramp steepness, however, decreases the precision with which a servo controller can regulate the velocity of the data-transducing head. Reducing data-access times requires an increase in the velocity at which the data-transducing head moves from its parked to its data-transducing positions. Increased head velocities make the need for precise velocity control more critical due to the above-noted potential for velocity excursions to damage the drive and the data-storage disk.
It is thus desirable to provide a disk drive with an improved ability to regulate the velocity of a data-transducing head while the head is moved between its parked and data-transducing positions. More specifically, the disk drive should have an improved ability to attenuate velocity variations in the head. The present invention addresses these goals.